Well, the best books I red were about things I thought of myself, but never expressed them, or not so clearly. The greatest thing about reading a book is finding pieces of yourself in it.
Write a book about what you're feeling, about what pissed you off, or about what you love. Write a book with characters that do something you wouldn't dare, write a book about a situation in your life, if only something big had happened that day... and the list goes on.
Sex sells. Yet no classic novelist had a sex-oriented book. Just write about what you feel. And ready your buttocks, since writing a book involves a lot of sitting down in front of a blank page!
Cheers!